There's several open source implementations available. The best options are probably Kaffe and SableVM. But a complete JRE is much more than just the byte-code compiler and interpreter - you need all the libraries too.

Libraries can (for the most part) be provided by Classpath but I don't think we have any way of saying (kaffe + classpath) == jre or anything like that. And then you also need to factor in that there's some longstanding issues with both kaffe and sablevm on alpha, although both have been getting much better the last year or so.

Kaffe used to fail about 143 tests out of 150 and is now down to a more manageable 7 failed tests iirc

In any case, Java on Alpha can be done but it's not something we can officially support at this point.